Comparator circuits compare two input voltages and amplify a voltage difference therebetween so as to output a comparison result signal. Particularly, latch comparator circuits, which have positive feedback circuits, perform the amplifying operation at a higher speed by means of the positive feedback circuit.
Various electronic circuits have comparator circuits. For example, an analog digital converter (ADC), which converts an analog input signal into a digital output signal, has a comparator circuit. This comparator circuit compares a sampled and held analog input voltage to a standard voltage so as to determine each bit. In a successive approximation ADC which sequentially detects a plurality of bits of the digital output signal from most significant bit (MSB) to least significant bit (LSB), the comparator circuit performs the determining operation a plurality of times. When the operating speed of the comparator circuit is slow, a converting speed of the analog-digital conversion (AD conversion) also becomes slow.
The successive approximation ADC is described in, for example, Japanese Patent Application Laid-Open No. 8-107354 and Japanese Patent Application Laid-Open No 2002-26731.
In the successive approximation ADC, a sample and hold circuit samples and holds an analog input voltage at a constant cycle, and the comparator circuit repeats a determining operation for determining whether the analog input voltage is higher or lower than a determination voltage the same number of times as the number of bits of the digital output. The successive approximation ADC is classified into a synchronous successive approximation ADC, which repeats the determining operation in a determining cycle obtained by dividing a holding cycle by the number of bits, and a nonsynchronous successive approximation ADC, which starts the determining operation on a next bit every time the determining operation on each bit is ended.